68 research outputs found
Electrical Transport Properties of Single-Layer WS2
We report on the fabrication of field-effect transistors based on single and
bilayers of the semiconductor WS2 and the investigation of their electronic
transport properties. We find that the doping level strongly depends on the
device environment and that long in-situ annealing drastically improves the
contact transparency allowing four-terminal measurements to be performed and
the pristine properties of the material to be recovered. Our devices show
n-type behavior with high room-temperature on/off current ratio of ~106. They
show clear metallic behavior at high charge carrier densities and mobilities as
high as ~140 cm2/Vs at low temperatures (above 300 cm2/Vs in the case of
bi-layers). In the insulating regime, the devices exhibit variable-range
hopping, with a localization length of about 2 nm that starts to increase as
the Fermi level enters the conduction band. The promising electronic properties
of WS2, comparable to those of single-layer MoS2 and WSe2, together with its
strong spin-orbit coupling, make it interesting for future applications in
electronic, optical and valleytronic devices
Piezoresistivity and Strain-induced Band Gap Tuning in Atomically Thin MoS2
The bandgap of MoS2 is highly strain-tunable which results in the modulation
of its electrical conductivity and manifests itself as the piezoresistive
effect while a piezoelectric effect was also observed in odd-layered MoS2 with
broken inversion symmetry. This coupling between electrical and mechanical
properties makes MoS2 a very promising material for nanoelectromechanical
systems (NEMS). Here we incorporate monolayer, bilayer and trilayer MoS2 in a
nanoelectromechanical membrane configuration. We detect strain-induced band gap
tuning via electrical conductivity measurements and demonstrate the emergence
of the piezoresistive effect in MoS2. Finite element method (FEM) simulations
are used to quantify the band gap change and to obtain a comprehensive picture
of the spatially varying bandgap profile on the membrane. The piezoresistive
gauge factor is calculated to be -148 +/- 19, -224 +/- 19 and -43.5 +/- 11 for
monolayer, bilayer and trilayer MoS2 respectively which is comparable to
state-of-the-art silicon strain sensors and two orders of magnitude higher than
in strain sensors based on suspended graphene. Controllable modulation of
resistivity in 2D nanomaterials using strain-induced bandgap tuning offers a
novel approach for implementing an important class of NEMS transducers,
flexible and wearable electronics, tuneable photovoltaics and photodetection.Comment: 12 pages, 4 figures in Nano Letters (2015
Electron and Hole Mobilities in Single-Layer WSe2
Single-layer transition metal dichalcogenide WSe2 has recently attracted a lot of attention because it is a 2D semiconductor with a direct band gap. Due to low doping levels, it is intrinsic and shows ambipolar transport. This opens up the possibility to realize devices with the Fermi level located in the valence band, where the spin/valley coupling is strong and leads to new and Interesting physics. As a consequence of its intrinsically low doping, large Schottky barriers form between WSe2 and metal contacts, which impede the injection of charges at low temperatures. Here, we report on the study of single-layer WSe2 transistors with a polymer electrolyte gate (PEO:LiCIO4). Polymer electrolytes allow the charge carrier densities to be modulated to very high values, allowing the observation of both the electron- and the hole-doped regimes. Moreover, our ohmic contacts formed at low temperatures allow us to study the temperature dependence of electron and hole mobilities. At high electron densities, a re-entrant insulating regime is also observed, a feature which is absent at high hole densities
Probing thermal expansion of graphene and modal dispersion at low-temperature using graphene NEMS resonators
We use suspended graphene electromechanical resonators to study the variation
of resonant frequency as a function of temperature. Measuring the change in
frequency resulting from a change in tension, from 300 K to 30 K, allows us to
extract information about the thermal expansion of monolayer graphene as a
function of temperature, which is critical for strain engineering applications.
We find that thermal expansion of graphene is negative for all temperatures
between 300K and 30K. We also study the dispersion, the variation of resonant
frequency with DC gate voltage, of the electromechanical modes and find
considerable tunability of resonant frequency, desirable for applications like
mass sensing and RF signal processing at room temperature. With lowering of
temperature, we find that the positively dispersing electromechanical modes
evolve to negatively dispersing ones. We quantitatively explain this crossover
and discuss optimal electromechanical properties that are desirable for
temperature compensated sensors.Comment: For supplementary information and high resolution figures please go
to http://www.tifr.res.in/~deshmukh/publication.htm
Disorder engineering and conductivity dome in ReS2 with electrolyte gating
Atomically thin rhenium disulphide (ReS2) is a member of the transition metal
dichalcogenide (TMDC) family of materials characterized by weak interlayer
coupling and a distorted 1T structure. Here, we report on the electrical
transport study of mono- and multilayer ReS2 with polymer electrolyte gating.
We find that the conductivity of monolayer ReS2 is completely suppressed at
high carrier densities, an unusual feature unique to monolayers, making ReS2
the first example of such a material. While thicker flakes of ReS2 also exhibit
a conductivity dome and an insulator-metal-insulator sequence, they do not show
a complete conductivity suppression at high doping densities. Using dual-gated
devices, we can distinguish the gate-induced doping from the electrostatic
disorder induced by the polymer electrolyte itself. Theoretical calculations
and a transport model indicate that the observed conductivity suppression can
be explained by a combination of a narrow conduction band and Anderson
localization due to electrolyte-induced disorder.Comment: Submitted versio
Valley Polarization by Spin Injection in a Light-Emitting van der Waals Heterojunction
The band structure of transition metal dichalcogenides (TMDCs) with valence
band edges at different locations in the momentum space could be harnessed to
build devices that operate relying on the valley degree of freedom. To realize
such valleytronic devices, it is necessary to control and manipulate the charge
density in these valleys, resulting in valley polarization. While this has been
demonstrated using optical excitation, generation of valley polarization in
electronic devices without optical excitation remains difficult. Here, we
demonstrate spin injection from a ferromagnetic electrode into a heterojunction
based on monolayers of WSe2 and MoS2 and lateral transport of spin-polarized
holes within the WSe2 layer. The resulting valley polarization leads to
circularly polarized light emission which can be tuned using an external
magnetic field. This demonstration of spin injection and magnetoelectronic
control over valley polarization provides a new opportunity for realizing
combined spin and valleytronic devices based on spin-valley locking in
semiconducting TMDCs.Comment: in Nano Letters (2016
Thickness-dependent mobility in two-dimensional MoS2
Two-dimensional (2D) semiconductors such as mono and few-layer molybdenum disulphide (MoS2) are very promising for integration in future electronics as they represent the ultimate miniaturization limit in the vertical direction. While monolayer MoS2 attracted considerable attention due to its broken inversion symmetry, spin/valley coupling and the presence of a direct band gap, few-layer MoS2 remains a viable option for technological application where its higher mobility and lower contact resistance are believed to offer an advantage. However, it remains unclear whether multilayers are intrinsically superior or if they are less affected by environmental effects. Here, we report the first systematic comparison of the field-effect mobilities in mono-, bi- and trilayer MoS2 transistors after thorough in situ annealing in vacuum. We show that the mobility of field-effect transistors (FETs) based on monolayer MoS2 is significantly higher than that of FETs based on two or three layers. We demonstrate that it is important to remove the influence of gaseous adsorbates and water before comparing mobilities, as monolayers exhibit the highest sensitivity to ambient air exposure. In addition, we study the influence of the substrate roughness and show that this parameter does not affect FET mobilities
Posttranscriptional regulation of UGT2B10 hepatic expression and activity by alternative splicing
The detoxification enzyme UDP-glucuronosyltransferase UGT2B10 is specialized in the
N-linked glucuronidation of many drugs and xenobiotics. Preferred substrates possess
tertiary aliphatic amines and heterocyclic amines such as tobacco carcinogens and
several anti-depressants and anti-psychotics. We hypothesized that alternative splicing
(AS) constitutes a mean to regulate steady state levels of UGT2B10 and enzyme
activity. We established the transcriptome of UGT2B10 in normal and tumoral tissues of
multiple individuals. Highest expression was in the liver, where ten AS transcripts
represented 50% of the UGT2B10 transcriptome in 50 normal livers and 44
hepatocellular carcinomas. One abundant class of transcripts involves a novel exonic
sequence and leads to two alternative (alt.) variants with novel in-frame C-termini of 10
or 65 amino acids. Their hepatic expression was highly variable among individuals,
correlated with canonical transcript levels, and was 3.5 fold higher in tumors. Evidence
for their translation in liver tissues was acquired by mass spectrometry. In cell models,
they co-localized with the enzyme and influenced the conjugation of amitriptyline and
levomedetomidine by repressing or activating the enzyme (40-70%; P<0.01), in a cell
context-specific manner. A high turnover rate for the alt. proteins, regulated by the
proteasome, was observed in contrast to the more stable UGT2B10 enzyme. Moreover,
a drug-induced remodelling of UGT2B10 splicing was demonstrated in the HepaRG
hepatic cell model, which favored alt. variants expression over the canonical transcript.
Our findings support a significant contribution of AS in the regulation of UGT2B10
expression in the liver with an impact on enzyme activity
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